Mix together the ingredients for the truffle sauce (except the butter) in a saucepan and bring to the boil. Reduce to a thin sauce consistency, stir in the butter and season with salt and pepper. Pass through a fine sieve and keep warm
For the fricassee, peel the celeriac and cut into small wedges. Clean the Jerusalem artichokes and cut to the same size as the celeriac. Blanch both in boiling salted water until al dente then strain and leave to dry
Meanwhile, heat a frying pan over a high heat and add the butter. When it begins to foam, fry the mushrooms then add the Jerusalem artichoke and celeriac. Cook until golden brown, season with salt and pepper and mix in the sliced truffle
Preheat the oven to 150°C/gas mark 2
Heat a griddle pan over a high heat. Rub the turbot portions lightly with olive oil and season with salt. Fry until coloured on one side then transfer to a baking dish with a splash of water and bake for 4–5 minutes, until the fish is cooked through
Serve the turbot with the truffle sauce and the vegetables on the side
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